The present invention relates to measurement tools and more particularly to tools for detecting and measuring the extent of misalignment between a machine shaft coupling bore and the shaft.
Measuring tools have traditionally been used in connection with various machine operations to perform different functions. For example, they have been employed for measuring eccentricity of work at progressive stages of machining, for centering boring devices and work pieces, one with respect to the other, and for checking alignment of the boring device and the work during the machining process. This class of tools is exemplified by U.S. Pat. No. 3,336,677 to Newton. There, the patentee was concerned with centering the tool bit with a pilot hole in the work piece in order to establish a reference point. That reference point was then used in the determination of the outer diameter of the hole desired in the work piece, and the drill bit was accordingly adjusted. The patentee employed a collet-and-mandrel fastener for securing the tool in the pilot hole of the work piece. Measurements taken to determine the final work piece hole diameter were made with the aid of the tool, but without reference to the fastener.
In another, and perhaps more critical, area of machine operations, measuring tools have been used to detect and measure misalignment between rotating machine shafts. Typically, rotating mechanical equipment includes a number of elements coupled by drive shafts. These shafts are supported by bearing assemblies and have end coupling bores designed to receive other mating rotating shafts. The end coupling bores may include simple keyway interlocks, intricate spline arrangements, or any other similar coupling mechanisms designed for torque transmission. In order to insure the most efficient use of rotary mechanical equipment, when two mating shafts are coupled, they must be designed so that their respective axes of rotation are coincident. Otherwise, during operation the misaligned shafts will generate destructive forces causing unbalance, axial movement, coupling wear, shaft fatigue, bearing failure and other related problems. The machine shaft is disposed in a housing having a mounting plate which includes a mounting bore concentric with the shaft, the mounting plate further including a surface which interfaces with the housing of a cooperating machine shaft.
One method used to determine shaft misalignment involves attaching a sensitive displacement measuring device, such as a dial indicator, to the shaft, and, while the shaft is rotated, traversing the mounting interface and the mounting bore. Traversing the mounting interface yields a determination of the lack of perpendicularity between the mounting interface and the shaft bearing axis of rotation. Traversing the mounting bore yields a measure of eccentricity. A major drawback to this procedure is that the measurements are taken with respect to the axis of rotation of the shaft, and not with respect to the shaft coupling bore axis. This is an important consideration since if a coupling bore's axis does not coincide with the axis of rotation of the shaft, due to machining or assembly errors, the abovementioned destructive forces will be generated.
Conventional methods of measurement using dial indicators attached to the shaft are limited to the determination of errors associated with the axis of rotation. For simple coupling bores having a keyway interlock, the measurement technique is relatively simple. However, for involute splined bores, no such simple method has heretofore been developed. Rather, methods utilizing such equipment as analog plotters have been employed, where individual splines within the couplings have been mapped and their position recorded on the plotter. Once all splines have been so mapped at several sections, the coupling axis of the splined bore may be mathematically determined. Clearly, this apparatus is difficult for the ordinary mechanic or machinist to understand. Moreover, the apparatus is much too cumbersome for use in restricted spaces such as aircraft engine compartments or ship engine rooms. Consequently, the standard inspection or measurement methods have been limited to the simpler determination of the shaft axis of rotation with respect to the housing mounting bore and mounting interface.